1. Technical Field
The present invention relates to a bending vibration piece and various electronic devices using a bending vibration piece.
2. Related Art
According to related arts, in various electronic devices such as digital still camera, video camera, navigation device, vehicle body attitude detection device, pointing device, game controller, mobile phone and head-mounted display, a piezoelectric vibration gyro using a bending vibration piece is widely used as a sensor for detecting physical quantities such as angular velocity, angular acceleration, acceleration and forces. Bending vibration pieces of various structures for piezoelectric vibration gyro are developed and proposed. For example, a double-side tuning fork-type bending vibration piece for angular velocity sensor including two forked members as a driven pair and detection pair connected by a base is known (see, for example, JP-A-64-31015).
Also, a double-side tuning fork-type rotational speed sensor in which a pair of drive excitation branches and a pair of detection pickup branches are connected to one side and the other side of a frame and in which the frame is connected via a suspension device to an attachment basal part arranged inside the frame via an aperture is known (see, for example, JP-A-7-55479). Vibration of the excitation branches on which Coriolis acceleration is exerted causes torsion on the frame with temporal change and this distortion is transmitted, vibrating the pickup branches. The attachment basal part is fixed to an attachment structure of a housing with an adhesive or the like. However, the suspension device between the frame and the attachment basal part minimizes the influence of a discrepancy between a coefficient of thermal expansion of a piezoelectric material of the bending vibration piece and a coefficient of thermal expansion of a material of the housing, on the vibration of the tuning forks.
When such a double-side turning fork-type bending vibration piece is downsized, the mass of vibration arms decreases. Therefore, there is a risk that the resulting Coriolis force decreases and causes a reduction in the sensitivity of the angular velocity sensor. Thus, a technique of realizing higher sensitivity of the angular velocity sensor by providing a groove at an end on a supporting part side of the vibration arms, thus reducing the rigidity thereof, then increasing the moment of a drive vibration arm in a driving mode and thus increasing the Coriolis force, or by providing a hole in the supporting part connecting the drive vibration arm and a detection vibration arm, thus lowering the rigidity thereof, and efficiently propagating vibration of the drive vibration arm to the detecting vibration arm, is known (see, for example, JP-A-2004-251663).
As a bending vibration piece for a piezoelectric vibration gyro that is not a double-side tuning fork-type, a so-called double-T structure is known (see, for example, JP-A-2003-166828). This bending vibration piece has a structure in which two drive vibration systems, each being substantially T-shaped and having a pair of drive vibration arms extending in opposite directions, are arranged with bilateral symmetry in relation to a detection vibration system including a pair of detection vibration arms extending in opposite directions from a central supporting part.
FIG. 9 schematically shows a typical example of a traditional double-side tuning fork-type bending vibration piece. In FIG. 9, a bending vibration piece 1 has a pair of drive vibration arms 3 extending parallel to each other on one side from a central supporting part 2, and a pair of detection vibration arms 4 extending parallel to each other on the side opposite to the drive vibration arms 3. On the supporting part 2, drive electrode pads 5 led out from drive electrodes (not shown) of the drive vibration arms 3 are arranged, one each, near a proximal end of each of the drive vibration arms. Moreover, on the supporting part, detection electrode pads 6 led out from detection electrodes (not shown) of the detection vibration arms 4 are arranged, two each, near a proximal end of each of the detection vibration arms.
As a predetermined AC voltage is applied to the drive electrodes of the drive vibration arms 3, the drive vibration arms 3 perform bending vibration in opposite directions to each other within an XY plane that is the same as main surfaces thereof. As the bending vibration piece 1 rotates on a Y axis in a longitudinal direction in this state of driving mode, a Coriolis force corresponding to an angular velocity thereof acts. The drive vibration arms 3 perform bending vibration in opposite directions to each other in Z-axis directions perpendicular to the main surfaces. By resonating with this bending vibration, the detection vibration arms 4 similarly perform bending vibration in opposite directions to each other in Z-axis directions. At this point, by taking out a potential difference generated between the detection electrodes of the detection vibration arms 4 from the detection electrode pads 6, the rotational and angular velocities of the bending vibration piece 1 or the like are found.
FIG. 10 schematically shows a modification of the double-side tuning fork-type bending vibration piece of FIG. 9. In a bending vibration piece 1′ of FIG. 10, a rectangular through-hole 7 is formed substantially at the center of the supporting part 2, as described in JP-A-7-55479 and JP-A-2004-251663. Thus, in detection mode, out-of-plane vibration of the drive vibration arms 3 is efficiently propagated to vibrate the detection vibration arms 4 and therefore detection sensitivity of the sensor is improved.
In the state of driving mode in which the drive vibration arms 3 performs in-plane vibration, a detection signal outputted from the detection electrode pads 6 is supposed to be 0 and should preferably be 0. However, in both cases of FIG. 9 and FIG. 10, it is found that when the bending vibration piece 1 is downsized, an error signal is outputted from the detection electrode pads 6 even if the bending vibration piece is not rotating on the Y axis. The output of an error detection signal in driving mode may deteriorate the detection sensitivity and accuracy of the angular velocity sensor.
Particularly when the bending vibration piece is downsized, the supporting part is downsized accordingly. However, the electrode pads formed on the surface of the supporting part need a certain area for connection with external wiring. Therefore, as the plane dimension of the supporting part decreases, the distance between the drive electrode pads and the detection electrode pads decreases and a large electrostatic coupling capacitance is generated between these electrode pads. Moreover, a drive current applied to the drive vibration arms in driving mode is considerably greater than a detection current outputted from the detection electrode pads in detection mode. This large electrostatic coupling capacitance is considered to be one of the causes of the occurrence of the error detection signal.
Since the supporting part is reduced in rigidity by the downsizing, vibration of the drive vibration arms in driving mode propagate to the detection vibration arms more easily. Such unwanted vibration of the detection vibration arms due to the mechanical vibration leakage from the drive vibration arms is considered to be another cause of the occurrence of the error detection signal.